Deck railing and baluster calculator
Balusters in a bay = (clear width − the largest gap) ÷ (baluster + gap), rounded down, plus one: a 68.5 in bay with 1.5 in balusters and gaps under 4 in takes 12, and 36 ft of railing takes 72.
Example: replace these measurements
Your material list
72 balusters+ 1 more item on the list
72 balusters
40 ft measured run
Your shopping list
- 72 balusters
- 8 total posts
Quantities and package details
- Railing length
- 36 ft
- Stair openings
- 4 ft
- Total posts
- 8
- End posts
- 2
- Corner posts
- 2
- Gate or stair posts
- 2
- Grade-change posts
- 0
- Line posts
- 2
- Bays
- 6
- Bay type 1 (6 bays)
- 68.5 in clear · 12 balusters · 3.88 in gaps · mark every 5.38 in
- Rail length
- 68.5 ft for 2 rails per bay
Gaps are equal within each bay. Check the finished gaps with a 4-inch sphere or block before fixing the last baluster.
The calculation
- floor((68.5 − 4) ÷ (1.5 + 4)) + 1 = 12 balusters; (68.5 − 12 × 1.5) ÷ 13 = 3.88 in gaps
- 72 balusters across 6 bays
Common questions
How far apart should deck balusters be?
Close enough that a 4-inch sphere cannot pass between them, the rule in the American Wood Council’s DCA 6 deck guide. The calculator spaces them evenly so every gap stays under your limit.
Source: AWC DCA 6 deck guide: guard requirements. Listed on the Sources page.
How many balusters per foot of railing?
A little over two. With 1 1/2-inch balusters and gaps just under 4 inches, each baluster and gap take about 5 1/2 inches.
How far apart should railing posts be?
The American Wood Council’s example deck guard shows posts at most 6 feet apart. Railing systems set their own spacing, so use yours.
Source: AWC DCA 6 deck guide: Figure 24, example guard detail. Listed on the Sources page.
When does a deck need a railing?
The American Wood Council’s guide says decks more than 30 inches above grade need a guard. Check your local code.
Source: AWC DCA 6 deck guide: guard requirements. Listed on the Sources page.
How to plan the quantity
Enter each side of the deck that needs a railing, measured along the edge. Runs that meet at a corner share the corner post. Leave a stair opening by entering its distance from the start of the run and the distance between the two stair posts.
Each run is split into equal bays no longer than your maximum post spacing. In each bay, the balusters are spaced evenly so the gap between them, and between a baluster and a post, stays under your limit. The list gives the gap and the on-center mark spacing, so you can mark every baluster before fixing it.
Rail length is the clear space between posts, times the rails in each bay. Cut lengths depend on how your rails fix to the posts.
Before you order
- This spaces balusters and counts parts. It does not size posts, choose connectors or approve a guard; AWC’s guide and your local code set how guard posts are attached.
- Stair railings follow the stair slope and need their own layout.
Before ordering, read the guide: How much tile do I need? Layout, cuts and boxes. Or choose another calculator.